The invention relates to a method and a device for optically measuring external threads, in particular pipes, according to the patent claims 1 and 4.
Pipes for example are provided with external threads for pipelines which convey especially fluids under pressure such as e.g. gas or oil. The pipes are hereby provided at their ends with an external thread and screwed together with a sleeve element having an internal thread.
In these pipelines under pressure, the demands on the screwed connections and thus the quality of the threads are high.
Such pipes are e.g. used as casings (protective pipes) and tubings (rising pipes) in oil and gas transport.
The typical outer diameter of these pipes amount to 60.3 to 177.8 mm on which threads are cut of up to 48 thread turns.
The quality check of the cut pipe threads requires a geometric measurement of the thread profiles in order to be able to compare them with the target specifications. The following values in particular are measured:
diameter of each thread turn,
pitch of the thread per inch,
width of the free edge between pipe end and thread beginning,
thread length in mm,
thread depth,
thread contour,
threads that were not cut through.
In the production line during cutting of the threads with a cycle time of 15 to 25 seconds for example, the quality check can be executed only with manual control of samplings.
Manual measurement of the threads is time-consuming, cost-intensive and inadequate for a comprehensive quality inspection. Moreover, there is a risk that the check of only samplings will not recognize in due time any deterioration of the thread quality so that waste and the threat of complaints increase.
DE 103 59 837 A1 discloses an optical process and an apparatus for automatic inspection of damage to threads of connecting elements, such as e.g. screws.
The apparatus includes a device for illuminating the screw being checked, with the illumination device being aligned in relation to the screw in such a way that the light beams radiate in substantial perpendicular relationship to a flank of the thread.
The apparatus further includes a collecting optical system for collecting the light beams reflected by the flank of the thread, wherein the collecting optical system is aligned in relation to the connecting element in such a manner that the substantially the light beams are captured which reflect perpendicular to the flank of the thread.
A measurement of pipe threads is, however, not possible with this apparatus as the thread profile is not detected.